In a typical visual search experiment, subjects see a string of letters flashed tachistoscopically, or on a computer-driven cathode ray screen. They must quickly indicate which of two previously defined target letters appeared in the display. Their reaction time can be recorded, as well as their accuracy. Previous investigators in human information processing have asked: are the letters in the display processed one at a time until the target is found (serially), or are they processed all at once (in parallel)? We believe that neither of these extreme alternatives is correct; instead, processing of successive letters overlaps in time. We propose to test this hypothesis in a series of experiments based on the visual search paradigm. Despite the many experiments already published, the basic parametric data needed to test the overlapping theory do not yet exist. By systematically varying target position and display size (over a wider range than has previously been used), we can provide these data. The assumption of overlapping processing of letters provides a plausible explanation for the incredible speed and efficiency of normal reading, as well as suggesting several possible ways in which such a system can break down to produce a reading disability.